drm/radeon: reintroduce radeon_dp_work_func content

Put back the radeon_dp_work_func logic.  It seems that
handling DP RX interrupts is necessary to make some
panels work.  This was removed with the MST support,
but it regresses some systems so add it back.  While
we are here, add the proper mutex locking.

Link: https://gitlab.freedesktop.org/drm/amd/-/issues/2567


Fixes: 01ad1d9c ("drm/radeon: Drop legacy MST support")
Reviewed-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: Lyude Paul <lyude@redhat.com>
Cc: stable@vger.kernel.org